Conventionally, in a magnetoelectric transducer having a magnetic sensor element of group III-V compound semiconductors, such as InSb or GaAs formed on an insulating layer or organic substance, such as polymer, Pb-Sn solder has been used for bonding lead wires and it has been impossible to package the magnetoelectric transducer on a printed wiring board through a process carried out at high temperatures which exceed a melting point of the solder. In recent applications of the magnetoelectric transducer to up-to-date apparatus such as a VTR and a CD player, any packaging process advantageous to mass production is carried out under the environment of high temperatures beyond the solder melting point. Therefore, it is necessarily required to adopt a process for soldering individual elements one by one to a printed wiring board. Accordingly, there has been desired a magnetoelectric transducer which will not be broken through a high temperature process beyond 200.degree. C.
One procedure for meeting, such a demand is such that electrodes are formed on a thin film of group III-V compound semiconductor on an organic insulating layer and wire bonding is effected by using wires of, for example, gold. Conventional techniques fail to permit practical realization of this procedure.
Reasons for this will be described. It has been attempted to form the electrode of the magnetoelectric transducer utilizing the group III-V compound semiconductor by a method in which after an ohmic contact layer is formed on the semiconductor layer, a metal layer of Au, Al or the like well suited for wire bonding is formed by, for example, vapor deposition, and the metal layer is heated to about 300.degree. to 400.degree. C. so that a thin wire of Au, Al or the like can be connected to the metal layer by compression bonding or by using ultrasonic compression bonding in combination. However, when applying the above method to a compound semiconductor film which is formed on a substrate having on its surface an organic insulating layer, two problems as below arise.
The first problem is that the temperature can not be raised sufficiently during bonding. When the temperature of the electrode portion is raised up to 300.degree. to 400.degree. C. as is effected usually, not only exfoliation occurs between the organic insulating layer and the semiconductor film, but also degradation such as carbonization of the organic insulating layer takes place. The exfoliation is due to the fact that the organic insulating layer and the compound semiconductor layer have different thermal expansion coefficients and hence thermal stress is concentrated at the interface between the organic insulating layer and compound semiconductor film when the temperature of the electrode portion is raised beyond 300.degree. C.
The second problem is that since the organic insulating layer is soft, it is more difficult to effect the compression bonding with concentration of the ultrasonic energy on the bonding portion, as compared to a crystal such as Si. Upon compression bonding, the application of a large amount of ultrasonic power is necessarily required with the result that the exfoliation occurs between the organic insulating layer and compound semiconductor film. For these reasons, the conventional wire bonding as applied to the compound semiconductor film on the organic insulating layer has been degraded in yield and its industrial application has been invalid.
At present, ultra-high temperature packaging typically represented by a reflow soldering process has also been demanded. Here, high temperature packaging means that the element is packaged on the printed wiring substrate or the like at about 180.degree. C. to 230.degree. C. and ultra-high temperature packaging means packaging at about 230.degree. C. to 260.degree. C.
Conventionally, the magnetoelectric transducer having a magnetic field sensing portion of group III-V compound semiconductor film formed on the substrate overlaying the organic insulating layer, that is, for example, an InSb Hall element commercially available for use in home-use electrical appliances has very excellent electrical characteristics and high sensitivity by magnetic amplification and the like but faces a problem that it has difficulties with packaging to the printed wiring board through an ultra-high temperature process carried out around 260.degree. C. such as reflow soldering. The formation of the magnetic field sensing portion of semiconductor film on the organic insulating layer provides good adhesion between the two, resulting in advantages that the substrate can be selected freely and that a high-sensitivity characteristic of the element can be obtained, and therefore it is widely used in the production of the magnetoelectric transducers.
Approaches to use a multi-layer electrode with the view of ensuring ohmic contact to the semiconductor layer, preventing electron migration in electrode metal and protecting the semiconductor thin film during solder bonding are referred to in, for example, U.S. Pat. No. 4,081,601 to Donald Dinella et al and U.S. Pat. No. 4,296,424 to the same applicants as the present application. In the patents, however, any organic insulating layer is not used as a base substrate and wire bonding is not taken into consideration.
Accordingly, an object of the present invention is to provide a magnetoelectric transducer having an electrode structure which permits rigid and highly-reliable ultrasonic wire bonding to be applied at high yields to a compound semiconductor thin film formed on an insulating substrate, i.e., a substrate made by itself of an organic insulating material or a substrate overlaying an organic insulating layer.
Another object of the present invention is to provide a highly reliable and highly sensitive magnetoelectric transducer wherein a magnetic field sensing portion formed of a group III-V compound semiconductor film on a substrate overlaying an organic insulating layer has a high heat resistance or high heat resisting property sufficient to withstand thermal shocks caused during ultra-high temperature packaging carried out at about 260.degree. C., thereby permitting automatic packaging.